1. Technical Field
The present disclosure relates to a DC/DC converter and a liquid crystal display.
2. Discussion of Related Art
A liquid crystal display may include a liquid crystal display part formed on a transparent glass substrate, a driver provided in the vicinity of the liquid crystal display part, and a DC/DC converter that supplies a driving voltage to the driver. The liquid crystal display part has a plurality of pixels arranged thereon in a matrix. Each pixel includes a thin film transistor, a pixel electrode electrically connected to a source area of the thin film transistor in series, a common pixel electrode facing the pixel electrode, and a liquid crystal interposed between the pixel electrode and the common pixel electrode. A horizontal scanning line is connected to a drain electrode of the thin film transistor of the pixel, and a vertical scanning line is connected to a gate electrode.
The demand for large-sized liquid crystal television displays for home and business use has increased. The demand for middle-sized, or small-sized liquid crystal displays, such as those sized at 20-inches or less, which are typically employed for portable phones, radio devices, digital cameras, and laptops, has also increased.
In the middle-sized or small-sized liquid crystal display, the area of peripheral circuits of a liquid crystal display part is reduced as compared to large-sized displays. The liquid crystal display part has to be received in a restricted space of a body of the portable terminal.
Technology has been developed to directly fabricate peripheral circuits on a transparent glass substrate of a liquid crystal display part to deal with the restricted space. The peripheral circuits include devices such as thin film transistors for pixels of the liquid crystal display part, a vertical driver that selectively drives vertical scanning lines, a horizontal driver that selectively drives horizontal scanning lines, and a DC/DC converter. A charge pump of the DC/DC converter includes a thin film transistor.
In the manufacturing process of a liquid crystal display, since a thin film transistor is manufactured on a transparent glass substrate that is vulnerable to high-temperature heat treatment, the thin film transistor employs a polycrystalline silicon film (low-temperature poly-silicon film), which is formed in a low-temperature process. A channel area, a source area, and a drain area of the thin film transistor are formed by using the low-temperature poly-silicon film. The thin film transistor includes a gate oxide layer formed on the surface of the channel area and a gate electrode formed on the surface of the gate oxide layer, in addition to the channel area, the source area, and the drain area. The gate oxide layer may be formed through a chemical vapor deposition (CVD) process.
However, the DC/DC converter and the liquid crystal display equipped with the DC/DC converter are manufactured without taking the following factors into consideration. Most middle-sized or small-sized liquid crystal displays are driven by power supplied from a battery instead of power from an external power source. The driving voltage of a liquid crystal display can be lowered to reduce power consumption. A signal or voltage boosted or dropped by the charge pump of the DC/DC converter is used inside the liquid crystal display. The DC/DC converter may be provided with a low voltage, such as 3V. A voltage boost charge pump supplied with low-voltage power includes a p-channel conductive-type thin film transistor. The transistor is used as a switching device to control current between a power source terminal and a voltage boost capacitor. However, since the channel area of the thin film transistor is formed by using a low-temperature poly-silicon film, and the position and the scale of a grain boundary are irregularly formed, a threshold voltage Vth of the thin film transistor is irregular. In addition, since the channel area includes a low-temperature poly-silicon film, the gate oxide layer formed on the channel area may have an irregular property, and the threshold voltage Vth of the thin film transistor may be irregular. Due to the irregularities, a potential difference Vgs between a gate voltage and a source area of the thin film transistor is insufficient and the capability of a driving current of the thin film transistor may be degraded. A voltage drop charge pump of the DC/DC converter includes an n-channel conductive-type thin film transistor used as a switching device that controls current between a grounding terminal and a voltage drop capacitor. Since the n-channel conductive-type thin film transistor includes a channel area formed by using a low-temperature poly-silicon film similar to the p-channel conductive-type thin film transistor, the capability of the driving current of the thin film transistor is degraded.
The above voltage boost and drop charge pumps degrade voltage boost and drop characteristics, and reduce a display rate in a liquid crystal display part of a liquid crystal display. Thus, there is a need for a DC/DC converter which can improve voltage boost or drop characteristics of a charge pump.